Method of preparing a semiconductor having controlled crystal orientation

ABSTRACT

A semiconductor device is disclosed. The semiconductor device has a crystalline silicon film as an active layer region. The crystalline silicon film has needle-like or columnar crystals oriented parallel to the substrate and having a crystal growth direction of ( 111 ) axis. A method for preparing the semiconductor device comprises steps of adding a catalytic element to an amorphous silicon film; and heating the amorphous silicon film containing the catalytic element at a low temperature to crystallize the silicon film.

This application is a Divisional of application Ser. No. 08/911,912,filed Aug. 15, 1997 now U.S. Pat. No. 5,882,960 which itself is aContinuation of Ser. No. 08/263,351, filed Jun. 21, 1994 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device which has a TFT(thin-film transistor) built onto an insulating substrate made of glassor the like, and to a method for its preparation.

2. Description of the Prior Art

Thin-film transistors (hereunder, TFTs) are known which employ thin-filmsemiconductors. These TFTs are constructed by forming a thin-filmsemiconductor on a substrate and using the thin-film semiconductor.These TFTs are used in various integrated circuits, but particularattention is being given to their use in electrooptical devices,especially as switching elements constructed for the respective pictureelements of active matrix-type liquid crystal displays and drivingelements formed in peripheral circuit sections.

The TFTs used in these devices generally employ thin-film siliconsemiconductors. Thin-film silicon semiconductors are largely classifiedinto two types: amorphous silicon semiconductors (a—Si) and siliconsemiconductors with crystallinity. Amorphous silicon semiconductors havea low preparation temperature, they may be relatively easily prepared bya vapor phase process, and they lend themselves well to masspreparation, for which reasons they are the most widely used type;however, their physical properties such as conductivity, etc. areinferior in comparison with those of silicon semiconductors withcrystallinity, and thus ardent attempts to establish new methods ofpreparing silicon semiconductor TFTs with crystallinity have been madein order to obtain more high-speed properties in the future. Ascrystalline silicon semiconductors, there are known polycrystallinesilicon, microcrystalline silicon, amorphous silicon which also containscrystalline components, and semi-amorphous silicon in an intermediatestate between crystalline and amorphous solids.

The following methods are known for obtaining thin-film siliconsemiconductors with crystallinity:

(1) Direct formation of a crystalline film at the time of its formation.

(2) Formation of an amorphous semiconductor film to which crystallinityis then imparted using the energy of laser light.

(3) Formation of an amorphous semiconductor film to which acrystallinity is then imparted by adding heat energy.

However, in method (1), it is technically difficult to form a uniformfilm with satisfactory semiconducting properties over the entire surfaceof the substrate, while another drawback is the cost, since with thefilm forming temperature being as high as 600° C. or above inexpensiveglass substrates cannot be used. In method (2), there is first theproblem of a small irradiation area of laser light, such as that from anexcimer laser which is the type most generally used at the present time,resulting in a low throughput, while the stability of the laser is notsufficient for uniform processing of the entire surface oflarge-surface-area substrates, and thus the method is thought to be anext-generation technique. Method (3) has a relative advantage overmethods (1) and (2) in that it is suitable for large surface areas, butit also requires high heating temperatures of 600° C. and above, andthus it is necessary to lower the heating temperature when usinglow-cost glass substrates. Particularly, in the case of liquid crystaldisplays presently in use there is a continuous drive toward large-sizescreens, and consequently the use of large-size glass substrates is alsonecessary. When large-size glass substrates are employed in this manner,shrinkage and warping which occur during the heating processindispensable to the preparation of the semiconductors result in lowerprecision of mask alignment, etc., and thus a major problem is inherent.Particularly, in the case of 7059 glass which is presently the mostwidely used type of glass, the warping point is 593° C. and consequentlymajor deformities are caused with the conventional processes for heatcrystallization. In addition to the problem of temperature, the heatingtime required for crystallization in the existing processes oftenreaches a few dozen hours or more, and thus a further shortening of thistime is necessary.

A greater problem is that fact that, since silicon thin films withcrystallinity prepared by these methods depend on coincidentalgeneration of nuclei and crystal growth therefrom, it has beenpractically impossible to control the particle size, orientation, etc.Many numerous attempts to control these have been made up to the presenttime, and as an example thereof there may be mentioned the patentedinvention described in Japanese Patent Application Publication HEI No.5-71993. Nevertheless, at present, methods such as the one described inthis patent publication still use coincidentally generated nuclei withina restricted range, and therefore at present control of the orientationof the film has not been complete, and there has been absolutely nocontrol of the particle size.

SUMMARY OF THE INVENTION

The present invention provides a means of overcoming the above mentionedproblems. More specifically, its purpose is to provide a process whichboth lowers the temperature and shortens the time required forcrystallization, in methods for the preparation of crystalline siliconsemiconductor thin films that involve heat crystallization of a thinfilm of amorphous silicon. It need not be mentioned, of course, that acrystalline silicon semiconductor prepared using the process provided bythe present invention has equal or superior physical properties incomparison with one prepared according to the prior art, and that it mayalso be used for the active layer region of a TFT.

More specifically, there is provided a new method of preparingcrystalline silicon thin-films which will supersede the conventionalmethod using coincidental nuclei generation, and it is a method ofpreparing crystalline silicon thin-films with sufficient productivity atrelatively low temperatures, which allows control of the particle sizeand a rather high degree of control of the orientation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the dependence of the orientation on the catalytic elementconcentration in a crystalline silicon film.

FIG. 2 shows a model for explanation of the crystallization mechanism.

FIG. 3 shows the steps of preparation in an example.

FIG. 4 shows the results of X-ray diffraction of a crystalline siliconfilm.

FIG. 5 shows the steps of preparation in an example.

FIG. 6 shows the results of X-ray diffraction of a crystalline siliconfilm.

FIG. 7 shows the steps of preparation in an example.

FIG. 8 shows the steps of preparation in an example.

FIG. 9 shows the relationship between film thickness and orientation ofa crystalline silicon film.

FIG. 10 shows the steps of preparation in an example.

FIG. 11 is an outline sketch relating in an example.

FIG. 12 shows the steps of preparation in an example.

FIG. 13 is a photograph showing the crystal structure of a silicon film.

FIG. 14 is a photograph showing the crystal structure of a silicon film.

FIG. 15 is a photograph showing the crystal structure of a silicon film.

FIG. 16 is an illustrative drawing showing crystal orientation of asilicon film.

FIG. 17 shows the concentration of nickel in a silicon film.

FIG. 18 is a sectional photograph of the front section of a siliconfilm.

FIG. 19 is an illustrative drawing showing the mechanism ofcrystallization of a silicon film.

DETAILED DESCRIPTION OF THE INVENTION

We the present inventors have carried out the following experiments andobservations regarding methods of promoting heat crystallization andmethods of controlling particle sizes and orientation, in order toovercome the problems involved in crystallization of amorphous siliconsuch as described above under

DESCRIPTION OF THE PRIOR ART

A description will first be given regarding a method of promoting heatcrystallization.

First, upon investigation of the mechanism for the formation of anamorphous silicon film on a glass substrate and the crystallization ofthe film by heating, as an experimental fact it was found that thecrystal growth begins at the interface between the glass substrate andthe amorphous silicon, and up to a certain film thickness it proceeds ina vertical columnar manner with respect to the surface of the substrate.

The above phenomenon is believed to be a result of the presence ofcrystal nuclei which become the base of crystal growth (seeds whichbecome the base of crystal growth) at the interface between the glasssubstrate and the amorphous silicon film, and of the growth of crystalsfrom the nuclei. These crystal nuclei are thought to be a contaminantmetallic element present in trace amounts on the surface of thesubstrate or crystal components on the glass surface (as indicated bythe expression “crystallized glass”, silicon oxide crystal componentsare present on the surface of glass substrates) or formed by stress.

Here, it was thought that the temperature of crystallization might belowered by more active introduction of crystal nuclei, and to confirmthis effect we attempted an experiment in which films of trace amountsof other metals were formed on substrates, and then thin films ofamorphous silicon were formed thereon, and heating was performed forcrystallization. As a result, in certain cases where a film containingseveral metals was formed on the substrate, a lowering in thecrystallization temperature was confirmed, and it was presumed thatcrystal growth had occurred with the foreign matter as the crystalnuclei. Thus we investigated in more detail the mechanism by which theuse of the various metal impurities resulted in a lowered temperature.

Crystallization may be thought of as comprising the two stages ofinitial preparation of nuclei and the crystal growth from the nuclei.Here, the rate of the initial preparation of the nuclei is found bymeasuring the time until the generation of minute dotted crystals at aconstant temperature, and this time was shortened in all cases of thinfilms in which the above mentioned metal impurity film had been formed,while it was also confirmed that the introduction of crystal nuclei hadthe effect of lowering the crystallization temperature. Also, quiteunexpectedly, when variations were made in the heating time for growthof the crystal grains after preparation of the nuclei, it was found thatthere was even a dramatic increase in the rate of crystal growth afterpreparation of the nuclei in case of crystallization of an amorphoussilicon thin films formed on films of certain types of metals. Adetailed explanation of this mechanism will be given later.

In any case it was found that, as a result of the two effects mentionedabove, when certain types of metals in trace amounts are used to form afilm on which a thin film made of amorphous silicon is formed and thenheated for crystallization, sufficient crystallization is achieved attemperatures of 580° C. and below for a period of about 4 hours, a factwhich could not be foreseen according to the prior art. Of the impuritymetals having such an effect, we selected nickel, as its effect is themost notable.

As an example of the degree of the effect provided by nickel, when on anuntreated substrate (Corning 7059), i.e. one on which no thin film oftrace nickel had been formed, a thin film of amorphous silicon formed bythe plasma CVD method was heated in a nitrogen atmosphere forcrystallization, a heating temperature of 600° C. required a heatingtime of 10 hours or more. However, when an amorphous silicon thin filmwas used on a substrate on which had been formed a thin film of tracenickel, a similar crystalline state could be obtained with only about 4hours of heating. Raman spectroscopy was used for the judgment of thiscrystallization. From this alone, it is clear that the effect of nickelis exceptional.

As is clear from the above explanation, if an amorphous silicon thinfilm is formed on a thin film formed using a trace amount of nickel,then it becomes possible to lower the crystallization temperature andshorten the time required for crystallization. Here, a more detailedexplanation will be provided with the assumption that the process isused for the preparation of a TFT. The description will be more specificlater, but the same effect is achieved not only when a thin film ofnickel is formed on the substrate, but also when it is formed on theamorphous silicon, and also with ion implantation or the like, andtherefore hereunder in the present specification all of these successivetreatments will be called “addition of a trace amount of nickel”.

First an explanation will be given regarding a method for the traceaddition of nickel. Clearly, for the trace addition of nickel, the sametemperature-lowering effect is provided either by a method of forming athin film containing a trace of nickel on a substrate and then formingthereon an amorphous silicon film, or by a method of first forming theamorphous silicon film and then forming the thin film containing a traceof nickel, and the formation of the films may be by sputtering, vapordeposition or plasma treatment, and thus it has been found that theeffect is accomplished regardless of the film-forming method. Plasmatreatment refers to a process in which a material containing a catalyticelement is used as the electrodes in a flat parallel-type or positiveglow column-type plasma CVD apparatus, and plasma is generated in anatmosphere of nitrogen, hydrogen, etc. for addition of the catalyticelement to the amorphous silicon film.

However, if a thin film containing a trace of nickel is formed on asubstrate, then rather than directly forming on a 7059 glass substrate athin film containing a trace of nickel, the effect is more notable if asilicon oxide thin film is first formed on the substrate and the thinfilm containing a trace of nickel is formed over it. One of the reasonsthat may be imagined for this is that direct contact between the siliconand the nickel is essential for the present low temperaturecrystallization, and it is thought that in the case of 7059 glass,components other than silicon might interfere with the contact orreaction between the silicon and the nickel.

Also, roughly the same effect was confirmed when the method used for thetrace addition of nickel did not involve formation of a thin film incontact with the top or bottom of the amorphous silicon, but ratheraddition by ion implantation.

Regarding the amount of the nickel, a lower temperature was confirmedwith addition of an amount of 1×10¹⁵ atoms/cm³ or more, but addition ofamounts of 1×10²¹ atoms/cm³ or more resulted in peaks of Raman spectrumwhose shape clearly differed in comparison with simple silicon, andtherefore it is thought that the practical range is 1×10¹⁵ atoms/cm³ to5×10¹⁹ atoms/cm³. Further, considering the physical properties of asemiconductor to be used as the active layer of a TFT, this amount mustbe kept to within 1×10¹⁵ atoms/cm³ to 1×10¹⁹ atoms/cm³.

Nevertheless, the presence of a large amount of an element such asmentioned above in a semiconductor is not preferred because it willreduce the reliability and electrical stability of a device using such asemiconductor.

In other words, the above mentioned crystallization-promoting elementsuch as nickel (in the present specification, thecrystallization-promoting element is referred to as “catalytic element”)is necessary for crystallization of the amorphous silicon, but ispreferably as little as possible contained in the crystallized silicon.In order to achieve this, an element having an extremely inactivetendency in the crystalline silicon is selected as the catalyticelement, and the amount of the catalytic element introduced is reducedto be as small as possible, that is, crystallization is carried out witha minimun amount of catalytic element. Regarding this amount, it hasbeen found that if the nickel concentration of the active layer is not1×10¹⁹ atoms/cm³ or lower then an adverse effect on the properties ofthe device will result. For this reason, the above dose of the catalyticelement must be strictly controlled when introduced.

In addition, when an amorphous silicon film was formed and then nickelwas added as the catalytic element by plasma treatment to prepare acrystalline silicon film, the following points have become apparent upondetailed investigation of the crystallization process, etc.

(1) When nickel is introduced onto the amorphous silicon film by plasmatreatment, the nickel penetrates through a considerable thickness intothe amorphous silicon film even prior to heat treatment.

(2) The initial generation of crystal nuclei occurs from the surface onwhich the nickel was introduced.

(3) Even if nickel is used to form the film on the amorphous siliconfilm by vapor deposition, crystallization occurs in the same manner aswith plasma treatment.

From the above points, it may be concluded that not all of the nickelintroduced by plasma treatment is effectively useful. That is, even if alarge amount of nickel is introduced, it is thought that some nickel ispresent which is not sufficiently useful. From this, it is believed thatthe point (surface) of contact between the nickel and the silicon is thekey to low temperature crystallization. Accordingly, it may be concludedthat the nickel must be minutely distributed in a discrete manner to thegreatest degree possible. That is, it may be concluded that “What isnecessary is that introduction of nickel with concentration as low aspossible within the possible range is made by its dispersion in adiscrete manner near the surface of the amorphous silicon film.”

As a method of introducing the trace amount of nickel into only theregion near the surface of the amorphous silicon film, i.e. a method fortrace addition of the crystallization-promoting catalytic element intoonly the region near the surface of the amorphous silicon film, theremay be mentioned the vapor deposition process; however, the vapordeposition process is disadvantaged by poor controllability anddifficulty in strictly controlling the amount of the catalytic elementintroduced.

Furthermore, since the amount of the catalytic element introduced mustbe as small as possible there often results the problem ofunsatisfactory crystallization, and thus it is important toappropriately adjust the amount of the catalytic element. As a means ofsolving these problems, the present inventors have invented a method ofadding the catalytic element using a solution, although a detaileddescription thereof is omitted in the present specification. By usingthis method, it has been revealed that the concentration of thecatalytic element may be controlled within a range of 1×10¹⁶ atoms/cm³to 1×10¹⁹ atoms/cm³. Also, as a result of investigation by the presentinventors, it has been found that as a catalytic element other thannickel which provides the same effect there may be used one or moreelements selected from the group consisting of Pd, Pt, Cu, Ag, Au, In,Sn, Pb, As and Sb.

The characteristics of crystal growth and crystalline form when a traceamount of nickel is added will now be discussed, with an additionalexplanation of the mechanism of crystallization assumed on the basisthereof.

As described above, it has been reported that if no nickel is added,then nuclei are randomly generated from the crystal nuclei on theinterface with the substrate, etc., and the crystal growth from thesenuclei is likewise random, and that crystals which are relatively (110)or (111) oriented are obtained depending on the method of preparation,and as a natural consequence crystal growth which is roughly uniformover the entire thin film is observed.

First, in order to determine the mechanism, an analysis was made with aDSC (differential scanning calorimeter). An amorphous silicon thin filmformed on a substrate by plasma CVD was placed with the substrate in aspecimen container, and the temperature was raised at a constant rate.Distinct exothermic peaks were observed at about 700° C., andcrystallization was observed. Naturally, this temperature shifted whenthe temperature-raising rate was changed, and when the rate was, forexample, 10° C./min crystallization started at 700.9° C. Next,measurements were made with three different temperature-raising rates,and the activation energies of the crystal growth after the initialgeneration of nuclei were determined by the Ozawa method. This resultedin a value of about 3.04 eV. Also, when the reaction rate equation wasdetermined by fitting with the theoretical curve, it was found to bemost easily explainable by a model of disorderly generation of nucleiand growth therefrom, thus confirming the propriety of the model inwhich nuclei are generated randomly from crystal nuclei on the interfacewith the substrate, etc., and crystal growth occurs from these nuclei.

Measurements completely identical to those mentioned above were alsomade with addition of a trace amount of nickel. This resulted ininitiation of crystallization at 619.9° C. with a temperature-raisingrate of 10° C./min, and the activation energy for the crystal growthdetermined by a series of these measurements was about 1.87 eV, thusshowing numerically as well the readiness of the crystal growth. Inaddition, the reaction rate equation determined by fitting with thetheoretical curve was closer to that of a one-dimensional interfacerate-determined model, suggesting crystal growth with orientation in acertain direction.

The data obtained from the above mentioned thermal analysis are providedin Table 5 below.

The activation energies shown in Table 5 were determined by measuringthe amount of heat released from the specimen during heating of thespecimen, and using the results for calculation by an analytical methodknown as the Ozawa method.

TABLE 5 Activation energy Activation energy Degree of (eV) (eV)crystallization Nickel added No nickel added 10% 2.04 2.69 30% 1.87 2.9050% 1.82 3.06 70% 1.81 3.21 90% 1.83 3.34 Average 1.87 3.04

The activation energies in Table 5 above are the parameters whichindicate the readiness of crystallization, and larger values indicatemore difficult crystallization, whereas smaller values indicate moreready crystallization. Judging from Table 5, the nickel-added specimenshave a lower activation energy as crystallization progresses. That is,as crystallization progresses, it more readily occurs. On the otherhand, it is shown that in the case of the crystalline silicon filmsprepared according to the prior art with no addition of nickel, theactivation energy rises as crystallization progresses. This indicatesthat crystallization becomes more difficult as it progresses.Furthermore, when the average values of the activation energies arecompared, the value for the silicon film crystallized by addition ofnickel is about 62% of that of the crystalline silicon film preparedwith no addition of nickel, which fact also suggests the readiness ofcrystallization of a nickel-added amorphous silicon film.

The following describes the results of observing the crystalline shapeof the film to which a trace amount of nickel was added, where 800 Åamorphous silicon was used as the starting film, using a TEM(transmission electron microscope). A characteristic phenomenon clearfrom the results of the TEM observation is that the crystal growth inthe nickel-added region differs from that in the surrounding sections.That is, a sectional view of the nickel-added region reveals that amoire or grating image-like stripe is present roughly perpendicular tothe substrate, and this leads to the conclusion that the added nickel orits compound formed with silicon functions as the crystallizationnucleus, and that the crystals grow roughly perpendicular to thesubstrate in the same manner as a film with no nickel added. Further, inthe regions around that in which nickel was added, there was observed astate in which needle-like or columnar crystal growth occurred in adirection parallel to the substrate.

A more detailed explanation of these phenomena will be given using thefollowing symbols which are basic to the field of crystallography.First, {hkl} is used to indicate all of the planes equivalent to the(hkl) plane. Likewise, <hkl> is used to indicate all of the axesequivalent to the [hkl] axis.

The results of morphological observation of the crystals around thenickel-added region will be discussed below. First, the fact thatcrystallization occurred in regions in which a trace of nickel had notbeen directly introduced was unexpected, but when the nickelconcentration in the trace nickel-added section, the section of lateralcrystal growth around it (hereunder referred to as “lateral growthsection”) and the distant amorphous sections (low temperaturecrystallization did not occur in the very distant sections) wasdetermined by SIMS (secondary ionic mass spectrometry), as shown in FIG.17, a lower concentration was detected in the lateral growth sectionsthan in the trace nickel-added section, and the amorphous sectionsshowed an amount of about one order less. In other words, it is thoughtthat the nickel had diffused over a rather wide area, and that thecrystallization in the regions around the nickel-added region was alsoan effect of the trace addition of nickel.

First, a surface TEM image of the area around the region of nickeladdition with amorphous silicon 800 Å thick is shown in FIG. 13. Thisfigure clearly shows characteristic needle-like or columnarcrystallization of uniform width in a direction generally parallel tothe substrate. Also, a layer with a different contrast than the otherportions of the crystal may be seen at the front section of the crystal,and from the results of subsequent high resolution TEM and TEM-EDX thissection was found to be NiSi₂, revealing the presence of a NiSi₂ layerperpendicular to the direction of the crystal growth. (This variesdepending on the film thickness, a fact that will be explained later).

Lateral growth generally parallel to the substrate was observed as faras a few hundred μm from the region of trace addition of nickel, and itwas also found that the degree of growth also increased in proportion toincreases in the time and temperature. As an example, with 4 hours ofcrystal growth at 550° C., about 20 μm of growth was observed. Next,FIG. 14 shows a TED pattern (electron beam diffraction image) for 3points in the above mentioned region of needle-like or columnar crystalgrowth. This TED pattern was taken from a direction perpendicular to thesubstrate. The pattern shows the crystalline structure of the siliconfilm. A look at this pattern clearly shows that it is very simple, suchthat it appears to be composed of single crystals or at most pairedcrystals, and the orientation of the crystals is very extremely uniform.From this pattern it is clear that the axial direction of the crystalresulting from lateral growth using the above mentioned 800 Å thickamorphous silicon film as the starting film, was the <111> direction.This relationship is shown in FIG. 16.

Based on the above experimental facts, the present inventors are of theopinion that the crystallization is promoted by the following mechanism.

First, considering the vertical growth, generation of nuclei occurs atthe initial stage of crystallization, and the activation energy at thistime is lowered because of the trace of nickel. This is self-evidentfrom the fact that the addition of the nickel causes crystallization tooccur at a lower temperature, and the reason therefor is thought to be,in addition to the effect of the nickel as foreign matter, the fact thatone of the intermetallic compounds (NiSi₂) composed of nickel andsilicon which are produced at a temperature lower than that ofcrystallization of the amorphous silicon acts as a nucleus forcrystallization because its lattice constant is close to that ofcrystalline silicon. Furthermore, this generation of nuclei occursalmost simultaneously over the entire surface of the region of nickeladdition, and thus the mechanism of the crystal growth is such that thegrowth occurs as a plane, and in such a case the reaction rate equationreflects a one-dimensional interface rate-determining step, withcrystals obtained having grown in a direction generally perpendicular tothe substrate. However, the crystalline axes are not completely uniformas a result of limitations on the film thickness, stress, etc.

Nevertheless, since the horizontal direction with respect to thesubstrate is more homogeneous than the perpendicular direction, theneedle-like or columnar crystals grow uniformly in a lateral directionwith the nickel-added section as the nucleus, and the direction of theplane of growth is <111>; for example, if an amorphous silicon film 800Å thick is used, the crystal growth is likewise the <111> direction.Obviously, in this case as well the reaction rate equation is presumedto be of a one-dimensional interface rate-determining type. As describedearlier, because the activation energy for the crystal growth is loweredby the addition of nickel, the rate of the lateral growth is expected tobe very high, and in fact it was.

An explanation will now be given regarding the electrical properties ofthe above sections of trace addition of nickel and of the surroundinglateral growth sections. Regarding the electrical properties of theregion of trace addition of nickel, the conductivity was about the samevalue as for a film with no nickel added, or a film heated for a fewdozen hours at about 600° C. Also, when the activation energy wasdetermined based on the temperature dependence of the conductivity, nobehavior seemingly attributable to the level of nickel was observed ifthe amount of nickel was about 10¹⁷ atoms/cm³ to 10¹⁸ atoms/cm³ asmentioned above. In other words, the experimental facts lead to thesupposition that within the above range of concentration, the film maybe used as the active layer of a TFT, etc.

In contrast, the lateral growth sections had a conductivity which wasone order or more higher in comparison with the region of trace additionof nickel, and this is a high value for silicon semiconductors withcrystallinity. This was thought to be due to the fact that, since thepath direction of the current matched the direction of lateral growth ofthe crystals, in the sections between the electrodes through whichelectrons pass the grain boundary was either few or virtuallynon-existent, and thus there was no contradiction with the results shownin the transmission electron micrographs. That is, it is conceivablethat because the migration of the carrier occurred along the grainboundary of the needle-like-or columnar-growing crystals, a conditionfor easy migration of the carrier had been created.

FIG. 15 is a TEM photograph showing the crystalline structure of thesilicon, with an enlarged view of the front section of the needle-likeor columnar crystal growth shown in FIG. 13 referred to above. In FIG.15, a black section may be seen at the front, and this section isclearly NiSi₂, as mentioned above. That is, nickel is concentrated atthe front of crystallization in which needle-like or columnar crystalgrowth occurred parallel to the substrate, and it is understood that thenickel concentration in the intermediate regions is low.

Here, as one of the effects of the present invention there may bementioned the fact that the degree of mobility of the carrier isincreased by roughly matching the direction generally along the crystalgrain boundary with the direction of migration of the carrier in thesemiconductor device (for example, a TFT). Furthermore, by avoiding thefront of the regions in which crystal growth occurs in a directionparallel to the substrate, and instead using the intermediate regions,i.e. the regions between the front of growth of the crystalline siliconfilm occurring in a lateral direction and the region of nickel addition,the crystalline silicon film with easy migration of the carrier as wellas low nickel concentration is employed.

The direction along the crystal grain boundary is the direction ofneedle-like or columnar crystal growth, and with a film thickness of 800Å (more correctly, the same has been found for greater film thicknessesas well) this direction of growth is the direction in which thecrystallinity is in the direction of the <111> axis, and further, asmentioned previously this direction is also the direction which has aselectively high conductivity with respect to the other directions (forexample, the direction perpendicular to the crystal growth). Also, anactual problem is that it is difficult to completely match the directionof crystal growth with the direction of flow of the carrier, and thecrystals also fail to grow over the entire surface in a uniformdirection. Accordingly, in practice the direction of crystal growth isdetermined on an average of directions. Also, that direction and thedirection of flow of the carrier are considered to match if they arewithin about ±20° of each other, and when an amorphous silicon film 800Å thick is used, it has been found to be clearly well within this range.

An explanation will now be provided regarding a method of controllingthe particle size and the orientation. The specimens in which catalyticelements were introduced for crystallization were subjected to X-raydiffraction, and the following items were investigated as theparameters.

Comparison between introduction of a catalytic element onto the surfaceof the amorphous silicon film and introduction thereof at its interfacewith the underlying film.

Comparison between the region of catalyst addition (referred to in thepresent specification as “vertical growth”) and the surrounding regionsof lateral growth.

Dependence on variation in the thickness of the amorphous silicon film.

Dependence on variation in the catalyst concentration.

In cases where the lateral growth process is employed, comparison forselection between a structure in which the above region of lateralgrowth is sandwiched at the top and bottom by silicon oxide and astructure with no silicon oxide on the top surface.

Further, to quantitatively evaluate the tendency observed when the abovementioned parameters were varied, the (111) orientation ratio wasdefined as shown in Equation 1 below, and the standard for a high (111)orientation was defined as a (111) orientation ratio of 0.67 or over.(For totally random powder, based on the above definition, the (111)orientation ratio is 0.33, and if the ratio is twice of this ratio orhigher, no problem is seen in referring to it as (111) orientation.)

Equation 1

Rate of (111) orientation=1 (constant)

Rate of (220) orientation=[relative strength of (220) to (111) forspecimen]/[relative strength of (220) to (111) for powder]

Rate of (311) orientation=[relative strength of (311) to (111) forspecimen]/[relative strength of (311) to (111) for powder] (111)Orientation ratio=[Rate of (111) orientation]/[Rate of (111)orientation+Rate of (220) orientation+Rate of (311) orientation]

From the results of (111) orientation ratio, results shown in Tables 1to 4 and FIG. 1 are obtained.

TABLE 1 Location of catalyst addition Orientation Particle size Siliconsurface Relatively random Uniform Interface with Strongly (111) Uniformsubstrate

TABLE 2 Method of growth Orientation Particle size Vertical growthRelatively random Uniform Lateral growth Strongly (111) Uniform

TABLE 3 Thin ← Film thickness → Thick Strong ← (111) orientation → Weak

TABLE 4 Presence of top surface oxide film Orientation Present Generally(111) Absent Variation due to film thickness

The methods of preparation were all the same except for the parameterslisted in the tables, while nickel was used as the catalytic element,the manner of addition of the nickel was from a solvent (hereunderreferred to as the liquid phase method), and in cases where there is noindication for lateral growth, vertical growth was employed byapplication of the solution on the silicon surface. However, for theexperiment of comparing the performance with the presence or absence ofa silicon oxide film on the surface in the lateral growth process, inorder to effect a lateral growth process with no silicon oxide on thetop surface there was used a nickel-added solution for SOG, such as OCDor the like, and unlike the other lateral growth processes, the OCD wasleft only in the region of direct addition (region of vertical growth)to create a structure wherein no silicon oxide was present on the regionfor lateral growth. Furthermore, the solid phase growth (also indicatedby SPC in the drawings) was induced by heating at 550° C. for 8 hours,and was followed by laser crystallization (the crystallinity may bedramatically increased by this complementary treatment) at 300mJ/cm^(2.)

The results of changing the location of addition of the catalyst aregiven in Table 1, which shows the specific tendency for totallydifferent orientation even with only a change in the location ofaddition. Virtually no dependence on the location of addition was foundfor the particle size, and when the particle sizes were measured atarbitrary locations, the width of distribution was found to be abouthalf that of the cases in which no catalytic element was added, andtherefore a uniform particle size had clearly been obtained.

Table 2 gives the results of changing the method of crystal growth, andit shows a comparison between a case where nickel was introducedthroughout the entire surface (vertical growth) and a case in which asilicon oxide film was formed on the amorphous silicon (silicon oxidecover), the silicon oxide was patterned to make openings for thecatalytic element, and lateral growth was induced from those openings.As a result, the vertical growth section was relatively random, whereasalmost all of the lateral growth sections covered with the silicon oxidefilm had a (111) orientation, although it depended on the filmthickness. (The dependence on film thickness is described later).

Table 3 shows the dependence on film thickness, and when experimentationwas conducted for film thicknesses of 300 Å to 5000 Å, in the lateralgrowth sections there was observed a clear tendency toward a stronger(111) orientation for smaller film thicknesses. A linearity wasdiscovered in the range of 400 Å to 800 Å roughly within the margin oferror, as shown in FIG. 9. Since the vertical growth section was randomfrom the start, no obvious tendency was found.

Table 4 shows the results of comparison of the effect of the presence orabsence of a surface silicon oxide film on the lateral growth process,and as mentioned previously, in the case of the specimens whichunderwent the lateral growth process with no silicon oxide on the topsurface, though the orientation changed depending on the film thicknessnone of the orientation was (111) orientation, as mentioned above,whereas the orientation of the crystalline silicon obtained by thelateral growth process with silicon oxide on the surface was strongly(111) orientation, and particularly, as FIG. 9 strongly suggests, at 800Å or less there was a rather strong (111) orientation. From this it isconcluded that the (111) orientation may be reinforced by making thefilm thickness 800 Å or less.

FIG. 1 shows a plot of the dependence of lateral growth on changes inthe nickel dose, with the horizontal axis showing the dose forliquid-phase addition using nickel acetate or nitrate, the left verticalaxis showing (111) orientation ratio and the right vertical axis showingthe proportion of the region of the silicon film crystallized bysolid-phase growth before laser crystallization. From this graph it isclear that it is possible to freely change the (111) orientation ratiofrom random to (111) orientation by varying the concentration of thecatalytic element. Furthermore, it is understood that these completelymatch the proportion changes in cases where solid-phase growth occurredprior to laser crystallization, and this was confirmed by observing thesame tendency in other cases in which the proportion of solid-phasegrowth which occurred prior to laser crystallization was varied bychanging the heating temperature and heating time instead of theconcentration.

Regarding the particle size, which is not shown in the drawings,observation of the particle size using an optical microscope (Atpresent, it is not clear whether the object is monocrystalline.)confirmed a reduction from 33 μm to 20 μm accompanying the doseincrease. Regarding the mechanism of the above experimental results, asfar as the orientations are concerned all of the effects may beexplained as the degree of influence of the silicon/silicon oxideinterface during solid growth. From this point of view, the explanationof the above phenomenon is as follows.

Regarding the results in Table 1, when a catalytic element is introducedat the interface with the substrate the orientation is alreadyinfluenced by the substrate at the time of nucleus generation, and atthis point the probability of (111) orientation is high. In comparison,when nuclei are generated at the top surface, random generation ofnuclei may be effected without any influence of the substrate. Thus, itis believed that these factors control the crystal growth throughout.

Regarding the results in Table 2, the vertical growth section has thesame mechanism as described above, and as regards the lateral growth,since the growth points grow while in contact with the substrate and thesilicon oxide cover, they are believed to be much more susceptible tothe influence thereof.

Regarding the film thickness dependency in Table 3, it is thought thatif the film thickness is increased, the proportion of the energy at theinterface with the underlying silicon oxide with respect to the totalfree energy is relatively lowered, and thus the (111)-orienting force isweakened.

Regarding the results of the two lateral growths (of films with andwithout a silicon oxide film on the top surface) in Table 4, it isthought that the film with the top surface covered with silicon oxidebecomes to be covered with silicon oxide on the top and the bottom, andto have the (111) orientation which stabilizes the interfaces. Incontrast, it is thought that the lateral growth process which occurswith no silicon oxide on the top surface has the interface contributionlowered to only half as much, thereby weakening the restraint onorientation to the same degree and thus exhibiting orientation otherthan (111). Incidentally, as mentioned above, in the lateral growthprocess occurring with no silicon oxide film on the top surface, therewas a clear correlation between the film thickness and the orientation.For example, when 500 Å amorphous silicon was used, strong (200) or(311) orientation was observed. In this regard, it may be concluded froma crystallographical analysis and from photographs such as the one shownin FIG. 18 that the crystal growth is occurring by a mechanism similarto the one in FIG. 19. That is, the crystal growth plane 501 or 506 isthe (111) plane, and though it is always constant, the angle which thisplane makes with the substrate is almost unconditionally defined by thefilm thickness. Consequently, if the film thickness is changed, forexample, with a film thickness of 800 Å, the apparent direction ofcrystal growth 504 and the crystal growth plane 501 are roughlyperpendicular, and the resulting orientation (orientation usually refersto the orientation of the direction perpendicular to the substrate) isobserved to be in a direction perpendicular to the axis <111>. However,with a film thickness of 500 Å, the crystal growth plane 506 and theapparent direction of crystal growth 505 are not perpendicular, andconsequently the orientation also changes. In other words, in theprocess of lateral growh with no silicon oxide on the top surface, theorientation may be controlled by varying the film thickness.

The results shown in FIG. 1 are easily explained by recognizing that theabove mentioned vertical growth proceeds at random, and the lasercrystallization exhibits (111) orientation. FIG. 2 shows a simplifiedversion of the mechanism therefor. In this figure, A is an examplewherein the dose of the catalytic element is small, there are few randomsections in which crystallization by solid-phase growth occurs beforelaser crystallization, and there are more sections of (111) orientationdue to laser crystallization; B is an example wherein almost all of thegrowth is solid-phase growth, and hence there are almost no sections of(111) orientation due to laser crystallization. To support the above, anexperiment was attempted in which the energy density and irradiationtime were varied during laser crystallization. It was thereby shown thatas the energy density and irradiation time were increased, the (111)orientation ratio became higher. This result suggests a directconnection between raising the proportion of laser crystallization andthe (111) orientation ratio.

As concerns the particle size, the above phenomenon may be on anassumption that the nucleus generation density is unconditionallydetermined by the dose of the catalytic element, not by the location,etc. of the catalytic element, which in turn determines the size towhich the crystal growth may occur.

In summary, the method of controlling the low temperaturecrystallization and the orientation are as follows.

First, using the method of adding a catalytic element, represented bynickel, at the surface by the liquid phase method, the crystallizationis effected as a combination of solid-phase growth and lasercrystallization. Thus, by addition of a trace amount of a catalyticelement, it becomes possible to lower the crystallization temperatureand dramatically reduce the time required.

For a film with a high (111) orientation

The lateral growth process is employed or the crystallization ratiobefore laser crystallization is lowered. By using this method it ispossible to adjust the (111) orientation ratio as desired within 0.67to 1. The method selected for lowering the crystallization ratio may bea method of reducing the dose of the catalytic element or varying theconditions of the solid-phase growth.

For a random film

Using the vertical growth process, the crystallization ratio beforelaser crystallization is increased. The method selected for increasingthe crystallization ratio may be by raising the dose of the catalyticelement or varying the conditions of the solid-phase growth.

For a film with an intermediate orientation

Using the vertical growth process, the crystallization ratio beforelaser crystallization is suitably adjusted. By using this process it ispossible to adjust the (111) orientation ratio as desired within 0.33to 1. The method selected for adjusting the crystallization ratio to asuitable value may be by varying the dose of the catalytic element orvarying the conditions of the solid-phase growth.

For a film with other orientations

Using the lateral growth process on a film with no silicon oxide on thetop surface, the film thickness is changed to control the orientation.Here, from the viewpoint of controllability the film thickness ispreferably varied within a range of about 800 Å to 300 Å. Withthicknesses above this range, the width of the columnar crystals wasless than the film thickness and there was a greater tendency towardrandomness, while with thicknesses less than 300 Å crystal growth wasdifficult.

The method of varying the particle size of the crystals may be thefollowing.

For a larger particle size, the concentration of the added catalyticelement is lowered.

For a smaller particle size, the concentration of the added catalyticelement is raised.

While controlling the dose of the above mentioned catalytic element, itis effective to control temperature and time of the solid-phase growth.However, the maximum degree to which the particle size may be increasedis unconditionally determined by the dose of the above mentionedcatalytic element.

According to the present invention, the most notable effect is achievedwhen nickel is used as the catalytic element, but other species ofcatalytic elements which may be used include Pt, Cu, Ag, Au, In, Sn, Pd,Pb, As and Sb. In addition, one or more elements selected from the groupconsisting of Group VII, IIIb, IVb and Vb elements may also be used.

Furthermore, the method of introducing the catalytic element is notlimited to the liquid-phase method using a solution such as an aqueoussolution, one in alcohol or the like, as a wide range of substancescontaining catalytic elements may be used. For example, metalliccompounds and oxides which contain a catalytic element may be used.

Finally, an explanation will be given regarding methods of applying thevarious properties mentioned above to a TFT. Here, the field ofapplication of the TFT is assumed to be an active matrix-type liquidcrystal display which uses the TFT as a driver for a picture element.

As described above, it is important to minimize the shrinkage of theglass substrates in recent large-screen active matrix-type liquidcrystal displays, and by using the process of adding a trace amount ofnickel according to the present invention, the crystallization ispossible at a sufficiently low temperature compared to the warping pointof glass, and thus it is a particularly suitable method. By followingthe present invention, the section using the conventional amorphoussilicon may easily be replaced by crystalline silicon by adding a traceamount of nickel thereto and conducting the crystallization at about 500to 550° C. for about 4 hours. Obviously, some variations will berequired to adapt to specific rules of design, etc., but the presentinvention may be carried out satisfactorily with the devices and theprocesses of the prior art, and thus its advantages are thought to beconsiderable.

EXAMPLES Example 1

This example illustrates the preparation of a silicon film with a high(111) orientation which involves selective provision of a 1200 Å siliconoxide film which is used as a mask for selective introduction of nickeland lateral growth.

The preparation process in this example is outlined in FIG. 3. First, onan amorphous silicon film (a 500 Å film prepared by plasma CVD) placedon a glass substrate (Corning 7059, 10 cm square) there is formed asilicon oxide film 21 which functions as a mask (silicon dioxide cover)to a thickness of 1000 Å or more, 1200 Å in this example. The inventorshave experimentally confirmed that a thickness of the silicon dioxidefilm 21 exceeding 500 Å does not cause any particular trouble, and it isbelieved that thinner films may be accepted as long as they have finequality.

The silicon oxide film 21 is then patterned as desired by a conventionalphotolithographical patterning process. Thereafter irradiation withultraviolet ray is conducted in an oxygen atmosphere for the formationof a thin silicon oxide film 20. The preparation of this silicon oxidefilm 20 is effected by irradiation with UV rays for 5 minutes in anoxygen atmosphere. Here an appropriate thickness of the silicon oxidefilm 20 is believed to be around 20-50 Å (FIG. 3(A)). Relating to theprovision of this silicon oxide film for improvement of the wettability,it may be effected appropriately due to the hydrophilic properties ofthe masking silicon oxide film alone in the case where the solution andthe pattern have a matching size. This case is, however, exceptional,and in most cases the use of the silicon oxide film 20 is recommendedfor security.

To the substrate in this state there is added dropwise 5 ml of anacetate solution which contains 100 ppm of nickel. At the same time spincoating is carried out for 10 seconds with a spinner at 50 rpm to form auniform aqueous film 14 on the entire surface of the substrate. Afterthis state is maintained for an additional one minute, spin drying isconducted for 60 seconds with a spinner at 2,000 rpm. It may be rotatedat 0 to 100 rpm on the spinner during that lapse (the additional oneminute)(see FIG. 3(B)).

Then heat treatment is effected for 8 hours at 550° (in a nitrogenatmosphere) for the crystallization of the amorphous silicon film 12.Here the crystal grows in a lateral direction from the region comprisingthe nickel-introduced section 22 to regions with no nickel introduced,as indicated by 23. The degree of the lateral growth under the presetconditions was about 30 μm. Thereafter the silicon oxide cover waspeeled off with buffer hydrofluoric acid, followed by lasercrystallization with a KrF excimer laser (248 nm) at a power density of300 mJ/cm².

The thus prepared silicon film was subjected to X-ray diffraction andfound to have a very high level of (111) orientation, with a (111)orientation ratio of 0.917. The results are shown in FIG. 4.

Example 2

This Example is for a case where entirely the same process as in Example1 was used, varying only the thickness of the amorphous silicon film totwo levels, 400 Å and 800 Å.

The results revealed that the (111) orientation ratio obtained withX-ray diffraction for the 400 Å specimen was about 1.0, showing it to bean almost completely (111)-oriented film, while that of the 800 Åspecimen was 0.720, or somewhat less (111) orientation as compared withthe 500 Å sample.

Example 3

This example is for a case where a crystallization-facilitatingcatalytic element is contained in an aqueous solution which is thenapplied on an amorphous film, which is then subjected to heat forcrystallization followed by irradiation with laser light for furtherimprovement of the crystallinity. This case corresponds to the verticalgrowth in the previous description, and may provide a film with arelatively random orientation.

The process as far as the introduction of a catalytic element (in thiscase, nickel was used) will be explained with reference to FIG. 5. Inthis example, Corning 7059 glass is employed as the substrate 11. Itssize is 100 mm×100 mm.

First, the amorphous silicon film 12 is formed to 100 to 1500 Å byplasma CVD or LPCVD. In this particular case, the amorphous silicon film12 is formed to a thickness of 500 Å by plasma CVD (FIG. 5(A)).

Then, to remove the dirt and spontaneous oxide film, treatment withhydrofluoric acid is carried out, after which an oxide film 13 of 10-50Å is formed. The spontaneous oxide film may be employed directly insteadof the oxide film 13 if the dirt is negligible.

The oxide film 13 is extremely thin and thus the exact thickness of thefilm is impossible to measure; nevertheless, it is believed to beroughly 20 Å. In this example, the oxide film 13 is formed byirradiation with UV light in an oxygen atmosphere. The film-formingcondition was UV irradiation for 5 minutes in an oxygen atmosphere.Thermal oxidation may be used as the process for the formation of theoxide film 13. Treatment with hydrogen peroxide may be effected as well.

The oxide film 13 is intended to facilitate the spreading of the acetatesolution over the whole surface of the amorphous silicon film; that is,for the improvement of the wettability, during the later step ofapplying the acetate solution which contains nickel. For example, if theacetate solution is applied directly on the surface of the amorphoussilicon film, then the amorphous silicon film repels the acetatesolution, and thus nickel cannot be introduced on the entire surface ofthe amorphous silicon film. In other words, uniform crystallization isimpossible to accomplish.

Next there is prepared an aqueous acetate solution which containsnickel. The concentration of nickel is adjusted to 25 ppm. A 2 mlportion of the resulting solution of an acetate is then added dropwiseto the surface of the oxide film 13 on the amorphous silicon film 12 forthe formation of an aqueous film 14. This state is maintained for 5minutes. Then a spinner is employed to conduct spin drying (2,000 rpm,60 seconds) (FIGS. 5(C), (D)).

The actual nickel concentration may be as low as 1 ppm or more, but thelevel was set to 25 ppm in this example in view of the desiredorientation. The use of a solution in a non-polar solvent, e.g. nickel2-ethyl-hexanoic acid in toluene as the solution makes it unnecessary touse the oxide film 13, allowing for direct introduction of a catalyticelement on the amorphous silicon film.

The step of applying the nickel solution may be repeated one to severaltimes to result in the formation of a nickel-containing layer which hasan average thickness of several to several hundred Å on the surface ofthe spin-dried amorphous silicon film 12. With this construction, thenickel in the layer diffuses into the amorphous silicon film and acts asa catalyst to facilitate the crystallization during the subsequentheating step. Here the layer is not always a complete film. Only oneapplication was conducted in this example.

After application of the solution mentioned above, the state ismaintained for one minute. The final concentration of the nickelcontained in the silicon film 12 may also be controlled by adjusting thelapse time, but the most significant control factor is the concentrationin the solution.

Heat treatment in a furnace is then conducted at 550° C. for 8 hours ina nitrogen atmosphere. As a result there may be provided a partiallycrystallized silicon film 12 formed on the substrate 11. Thecrystallization ratio at this stage was determined to be 98.84% bycomputer-aided image analysis.

The above heat treatment may be carried out at a temperature of 450° ormore, although the lower the temperature, the longer the heating time,which leads to a lower production efficiency. At 550 C. or more caremust be taken not to cause trouble due to the level of heat resistanceof the glass substrate used as the substrate.

In this example there is illustrated a method whereby a catalyticelement is introduced on the amorphous silicon film, but another methodmay be employed as well which introduces a catalytic element beneath theamorphous silicon film. According to the latter method, however, it isnoted that an extremely high level of (111) orientation is attained, asdescribed above. To the partially crystallized silicon film 12 obtainedby the heat treatment, there are given several shots of KrF excimerlaser light (wavelength: 248 nm, pulse width: 30 nsec.) at a powerdensity of 200 to 350 mJ/cm², in this particular example, one shot at300 mJ/cm², in a nitrogen atmosphere for complete crystallization of thesilicon film 12. This process may be carried out by irradiation with IRlight, as mentioned above.

The orientation of the thus prepared crystalline silicon film wasmeasured by X-ray diffraction. The results are shown in FIG. 6. Thepeaks of (111), (220) and (311) were clearly observed, and the (111)orientation ratio was calculated to be 0.405 based on this observation,proving that a desired random oriented film had been obtained.

Example 4

This example is a modification of Example 3 wherein the concentration ofthe salt of nickel, a catalytic element, has been changed into 1 ppm.The other conditions are the same as in Example 3. This constructionenables the particle size of each crystal to be enlarged. In thisexample experiments were conducted under two solid growth timeconditions of 4 and 16 hours.

Microscopic observation of the film after the heat treatment revealedthat the sample with the lowered concentration of the nickel salt andsubjected to solid growth for 4 hours had a greater proportion ofamorphous silicon and fewer numbers of crystal nuclei comprisingcrystalline silicon. Next the laser-crystallized specimen was subjectedto secco etching, followed by observation with SEM. As a result, it hasbeen found that with the lower concentration in the solution as in thepresent example the size of the respective crystal grains may beenlarged as compared with the case of Example 2.

In addition, by subjecting the laser-crystallized specimen to X-raydiffraction, there was formed a (111)-oriented film with a (111)orientation ratio of 0.730, from the specimen which had undergone solidgrowth for 4 hours. On the other hand, the orientation ratio of thespecimen obtained by solid phase growth for 16 hours was as low as about0.4, and the film was random.

Example 5

This example is for a case where the crystalline silicon film preparedby using the method of the present invention was employed to provide aTFT. The TFT of this example may be used as part of the driver circuitor picture element of an active matrix type liquid crystal display. Inthis connection, it need not be mentioned that the range of applicationof the TFT embraces not only liquid crystal displays, but also so-calledthin-film integrated circuits. The process for the preparation accordingto this example is outlined in FIG. 7. First an underlying silicondioxide film (not shown) is formed on the glass substrate 11 to athickness of 2000 Å. This silicon oxide film is provided to prevent thediffusion of impurities from the glass substrate.

Then a 500 Å-thick film of amorphous silicon is formed in the samemanner as in Example 1. Thereafter treatment with hydrofluoric acid wasconducted to remove the spontaneous oxide film, after which a thin oxidefilm was formed to a thickness of about 20 Å by irradiation with UVlight in an oxygen atmosphere. This process for preparing the thin oxidefilm may be replaced by treatment with hydrogen peroxide or thermaloxidation.

A solution of an acetate which contained 25 ppm of nickel was applied onthe film which was then allowed to stand for 1 minute, after which itwas subjected to spin drying with a spinner. Thereafter the silicondioxide films 20 and 21 (FIG. 3(A)) were removed with bufferhydrofluoric acid, followed by heating at 550° C. for 8 hours for thecrystallization of the silicon film. (The foregoing procedures are thesame as in the preparation process shown in Example 1).

The above heat treatment provides a silicon film comprising amorphousand crystalline components in admixture. The crystalline components makeup regions wherein crystal nuclei are present. Further 200-300 mJ/cm²,in this particular example 300 mJ/cm², of KrF excimer laser light wasirradiated to improve the crystallinity of the silicon film. During thisprocess for irradiation with the laser the substrate is heated to about400° C. This process is helpful to further improve the crystallization.

Then the crystallized silicon film was patterned to form a island region104. This island region 104 constitutes an active layer of the TFT.Thereafter a thin silicon oxide film 105 of thickness 200 to 1500 Å,specifically 1000 Å in this example, was formed. This silicon oxide filmalso functions as a gate-insulating film (FIG. 7(A)).

Care should be taken during the preparation of the above silicon oxidefilm 105. Here, the starting material TEOS was decomposed and depositedtogether with oxygen by RF plasma CVD at a substrate temperature of 150to 600° C., preferably 300 to 450° C. The pressure ratio of TEOS tooxygen was set 1:1 to 1:3, while the pressure and the RF power was setto 0.05 to 0.5 torr and 100 to 250 W, respectively. Alternatively, TEOSwas used as the starting material which underwent low pressure oratmospheric CVD at a substrate temperature of 350 to 600° C., preferably400 to 550° C., with ozone for the formation of the film. The formedfilm was annealed at 400 to 600° C. for 30 to 60 minutes in an oxygen orozone atmosphere.

The formed film may be subjected directly to irradiation with KrFexcimer laser light (wavelength: 248 nm, pulse width: 20 nsec) or anyother light of the same power to facilitate the crystallization of thesilicon region 104. Particularly, RTA (rapid thermal annealing) withinfrared rays selectively heats the silicon without heating the glasssubstrate, thereby reducing the interface level at the interface betweenthe silicon and the silicon oxide film, and thus is useful to prepareinsulated-gate type field-effect semiconductor devices.

Then an aluminum film of thickness 2,000 Å to 1 μm was formed byelectron-beam evaporation and patterned to form a gate electrode 106.The aluminum may be doped with 0.15 to 0.2% by weight of scandium (Sc).Next the substrate was dipped in a solution in ethylene glycolcontaining 1 to 3% tartaric acid at a pH of 7 for anodization usingplatinum as the cathode and the aluminum gate electrode, as the anode.For the anodization, first the voltage was increased to 220 V at aconstant current, and this condition is maintained for 1 hour tocomplete the process. In this example 2 to 5 V/min. was appropriate asthe rate of the voltage increase under a condition of constant current.There was thus formed the anodic oxide 109 having a thickness of 1500 to3,500 Å, e.g. 2,000 Å (FIG. 7(B)).

Thereafter ion doping (or plasma doping) was utilized to implant animpurity (phosphorus) into the island silicon film of each of the TFTswith the gate electrode member as the mask, in a self-aligning manner.The gas used for the doping was phosphine (PH₃). The dose is 1 to 4×10¹⁵cm⁻².

Then, as shown in FIG. 7(C), the film is irradiated with KrF excimerlaser light (wavelength: 248 nm, pulse width: 20 nsec.) to improve thecrystallinity of portions in which crystallinity had deteriorated due tothe introduction of the above impurity. The energy density of the laserlight is 150 to 400 mJ/cm², preferably 200 to 250 mJ/cm². Thus there areformed N-type impurity regions 108 and 109. The sheet resistance ofthese regions was 200 to 800 Ω/▪.

In this step, the laser light may be replaced by any other type of lightwhich is as powerful as laser light including so-called RTA (rapidthermal annealing) (or RTP: rapid thermal process) whereby a sample isheated to 1000 to 1200° C. (temperature of the silicon monitor) in ashort time with a flash lamp.

Then on the entire surface, there is formed a 3,000 Å thick siliconoxide film as the interlayer insulator from TEOS as the startingmaterial by plasma CVD with oxygen or by low pressure CVD or atmosphericCVD with ozone. The substrate temperature is set to 250 to 450° C., forinstance 350° C. The formed silicon oxide film is then mechanicallypolished to provide the surface with evenness (FIG. 7(D)).

Thereafter, as shown in FIG. 7(E), etching is made in the interlayerinsulator 110 to create a contact hole in the source/drain of the TFTand chrome or titanium nitride wirings 112 and 113.

Finally, annealing is conducted in hydrogen at 300 to 400° C. for 1 to 2hours for the completion of hydrogenation of the silicon. The TFT isthus completed. Many TFTs prepared at the same time are arranged to setup an active matrix-type liquid crystal display. The TFT hassource/drain regions 108/109 and a channel-forming region 114. Inaddition 115 denotes an electrical junction NI.

With the construction according to the present example the concentrationof nickel in the active layer is assumed to be about 3×10¹⁸ atoms/cm³ orless, more specifically 1×10¹⁶ atoms/cm³ to 3×10¹⁸ atoms/cm³.

The TFT prepared in this example has a mobility of 75 cm^(2/)Vs or morefor N channels. Further it was confirmed to have satisfactory propertieswith a small V_(th). Additionally the mobility was confirmed to rangewithin ±5%. This lowered variation is believed to be due to the randomorientation which does not cause anisotropy of the operatingcharacteristics of the device. Though 100 cm²/Vs or more may easily beestablished for N-channel types with laser light alone, the variationbecomes large and the uniformity observed in this example cannot beestablished.

Example 6

This example is a modification of the construction in Example 5, whereinthe nickel concentration has been changed to 1 ppm, and the crystalparticle size has been increased. The rest of the construction isexactly as in Example 5.

This resulted in a degree of mobility for the N channel of 150 cm²/Vs orgreater. This is thought to be the effect of the larger crystal particlesize. However, there were variations in the mobility of about ±30%, andthus the uniformity was not so high. The reason for this is not clear,but it is presumed to be that since it had some degree of (111)orientation, there was some possibility of the occurrence of anisotropyin the device.

Example 7

In this example, nickel is selectively introduced as shown in Example 2,and an electronic device is formed using the region of crystal growth ina lateral direction (parallel to the substrate) from the section ofintroduction. When employing such a construction, the nickelconcentration in the active layer regions of the device may be furtherlowered for a very desirable construction from the point of view ofelectrical stability and reliability of the device. In addition, bygiving the amorphous silicon film a film thickness of 400 Å it ispossible to obtain a film with almost total (111) orientation.

FIG. 8 shows the steps of preparation for this example. First, asubstrate 201 is washed, and an underlying film of silicon oxide isformed to a thickness of 2000 Å by the plasma CVD method using TEOS(tetraethoxysilane) and oxygen as the starting gas. Also, the plasma CVDmethod is used to form an intrinsic (type I) amorphous silicon film 203which has a thickness of 300 to 1500 Å, in this example 400 Å. Next, asilicon oxide film 205 is formed by the plasma CVD method in acontinuous manner to a thickness of 500 to 2000 Å, e.g. 1000 Å. Thesilicon oxide film 205 is then selectively etched to form a region ofexposed amorphous silicon 206.

A solution (here an acetate solution) containing elemental nickel as thecatalytic element to promote crystallization is then applied by themethod indicated in Example 2. The nickel concentration in the acetatesolution is 100 ppm. The detailed order of steps and the conditions areotherwise the same as indicated in Example 1. This process may also beaccording to the method given in Example 5 or Example 6.

After this, heat annealing is performed for 8 hours at 500 to 620° C.,e.g. 550° C., in a nitrogen atmosphere, and a silicon film 203 iscrystallized. The crystallization is promoted starting from the region206 where the nickel and silicon film are in contact, with the crystalgrowth in a direction parallel to the substrate, as indicated by thearrow. In the figure, region 204 is the section of crystallization withdirect addition of nickel, and region 203 is the section ofcrystallization in a lateral direction. The lateral crystallizationindicated by 203 is about 25 μm (FIG. 8(A)).

After the step of crystallization by the above heat treatment, thecrystallinity of the silicon film 203 is further promoted by irradiationwith laser light. This step is exactly the same as in Example 1, but inorder to perform the laser crystallization without removing the siliconoxide film 205, in this example, the crystallization was carried out at350 mJ/cm², an even higher energy than in Example 1.

Next, the silicon oxide film 205 is removed. At the same time, the oxidefilm formed on the surface of the region 206 is also removed. Thesilicon film is patterned by dry etching to form an island active layerregion 208. Here, the region indicated by 206 in FIG. 8(A) is the regioninto which nickel was directly introduced, and it has a highconcentration of nickel. As expected, a high concentration of nickel wasalso found at the front of the crystal growth. In these regions, thenickel concentration was clearly higher than in the middle regions.Consequently, in this example, these high nickel concentration regionswere not allowed to overlap the channel forming regions in the activelayer 208.

After this, the surface of the active layer (silicon film) 208 isoxidized by allowing it to stand for one hour in a 100% by volume watervapor-containing atmosphere at 10 atmospheres, and 500 to 600° C.,typically 550° C., to form a silicon oxide film 209. The thickness ofthe silicon oxide film is 1000 Å. After formation of the silicon oxidefilm 209 by thermal oxidation, the substrate is kept in an ammoniaatmosphere (1 atmosphere pressure, 100%) at 400° C. Infrared light witha peak at a wavelength of 0.6 to 4 μm, for example 0.8 to 1.4 μm, isirradiated onto the substrate in this state for 30 to 180 seconds, fornitriding of the silicon oxide film 209. In this situation theatmosphere may be mixed with 0.1 to 10% HCl. (FIG. 8(B))

Next, a film of aluminum (containing 0.01 to 0.2% scandium) is formed bythe sputtering method to a thickness of 3000 to 8000 Å, for example 6000Å. This aluminum film is then patterned to form a gate electrode 210(FIG. 8(C)).

Then, the surface of this aluminum electrode is subjected to anodizationto form an oxide layer 211 on the surface thereof. This anodization iscarried out in an ethylene glycol solution containing 1 to 5% tartaricacid. The thickness of the resulting oxide layer 211 is 2000 Å. Sincethis oxide 211 attains the thickness forming the offset gate region inthe following ion doping step, the length of the offset gate region maybe determined in the above anodization step (FIG. 8(D)).

Next, the ion doping method (or the plasma doping method) is used to addan impurity (here, phosphorus) which provides N-conductive type in aself-matching manner to the active layer region (comprising thesource/drain and channel), using the gate electrode section, i.e. thegate electrode 210 and the oxide layer 211 around it, as a mask.Phosphine(PH₃) is used as the doping gas, and the acceleration voltageis 60 to 90 kV, e.g. 80 kV. The dose is 1×10¹⁵ to 8×10¹⁵ cm⁻², e.g.4×10¹⁵ cm⁻². As a result it is possible to form N-type impurity regions212 and 213. As is also clear from the drawings, there is an offsetcondition of a distance x between the impurity region and the gateelectrode. This offset condition is particularly effective from theviewpoint of reducing the leak current (also called the off-current)when reverse voltage (minus for an N-channel TFT) is applied to the gateelectrode. In particular, a TFT according to this example in which thepicture element of an active matrix is controlled preferably has a lowleak current so that the charge accumulated in the picture elementelectrode does not escape, for a more satisfactory image, and thereforeproviding the offset is effective.

Annealing is then performed by irradiation of laser light. The laserlight used is from a KrF excimer laser (wavelength: 248 nm, pulse width:20 nsec), but other lasers may be used. The conditions of the laserlight irradiation are an energy density of 200 to 400 mJ/cm², forexample 250 mJ/cm², and irradiation with 2 to 10 shots, for example 2shots per location. A greater effect is achieved by heating thesubstrate to about 200 to 450° C. at the time of laser irradiation (FIG.8(E)).

Next, a silicon oxide film 214 of thickness 6000 Å is formed by theplasma CVD method as an interlayer insulator. Also, the spin coatingmethod is used to form a transparent polyimide film 215, to flatten thesurface.

Also, contact holes are formed in the interlayer insulators 214, 215,and TFT electrodes/wiring 217, 218 are formed by a multi-layer film ofmetal materials, such as titanium nitride and aluminum. Finally,annealing is performed in a hydrogen atmosphere at one atmospherepressure, at 350° C. for 30 minutes, thus completing the picture elementcircuit of the active matrix with a TFT (FIG. 8(F)).

The TFT prepared in this example has a high degree of mobility, and thusit may be used in the driver circuit of an active matrix-type liquidcrystal display. Specifically, a mobility of 250 cm²/Vs or greater wasachieved in the N-channel. It is presumed that this high mobility isattributed to reduction of the potential barrier of the grain boundarywhich is attributed to the very high degree of orientation of crystal.

Example 8

This example is a case in which the lateral growth method in Example 7was changed to a method employing OCD. That is, the silicon oxide film205 of thickness 500 to 2000 Å, e.g. 1000 Å which was continuouslyformed subsequent to the formation of the 500 Å intrinsic (type I)amorphous silicon film 203 was omitted, and instead a nickel-added SOGfilm, in this case an OCD Type-2 non-doped material Si-59000-SG, productof Tokyo Ohka Kogyo Co.,Ltd. was used to form a substance containing anickel compound. Before this film was formed, the surface was exposed toozone to form a very thin oxide film, and then the OCD was formed.

Prebaking at 80° C. and 150° C. was then effected, followed by curing at250° C. If this curing temperature is too high special attention will benecessary since nickel will already disperse in the amorphous siliconduring this step. Furthermore, the very thin oxide film produced by theozone acts as a barrier against dispersion in the curing step, and if itis absent special attention will be necessary since the nickel willdisperse even at 250° C.

Next, a prescribed patterning is effected. For this patterning, the maskin Example 7 was used and positive-negative inversion was performed witha resist. Regarding the etching after patterning, a dry rather than wetprocess is preferred because the etching rate of OCD is extremely fast.

The following steps are the same as in Example 7 and thus thedescription thereof is omitted. The characteristics of the resulting TFTwere almost the same as the one in Example 7.

When the gate section of the TFT was peeled off to determine theorientation of the active layer section below it by electrondiffraction, it was found that almost the entirety thereof had a (200)orientation.

Example 9

This Example is an case of forming a complementary integrated circuitincluding a P-channel type TFT (called a PTFT) and a N-channel type TFT(called an NTFT) each made using a crystalline silicon film on a glasssubstrate. The construction of this Example may be used in a switchingelement for a picture element electrode and a peripheral driver circuitof an active-type liquid crystal display, or an image sensor or otherintegrated circuit.

A sectional view showing the steps of preparation for this example isgiven in FIG. 10. First, an underlying film 302 of silicon oxide isformed on a substrate (Corning 7059) 301 to a thickness of 2000 Å by thesputtering method. Next, a mask 303 is provided which is a metal mask,or a silicon oxide film or the like. This mask 303 provides aslit-shaped exposure of the underlying film 302 at the region indicatedby 300. That is, when FIG. 10(A) is viewed from the top, the underlyingfilm 302 is exposed with a slit shape, while the other sections thereofare masked.

After the above mask 303 is provided, a nickel silicide film (chemicalformula: NiSi_(x), where 0.4≦×≦2.5, e.g., ×=2.0) is selectively formedon the region 300 by the sputtering method to a thickness of 5 to 200 Å,for example, 20 Å.

Next, an intrinsic (type I) amorphous silicon film 304 of thickness 500to 1500 Å, for example 1000 Å, is formed by the plasma CVD method. Thisis then crystallized by annealing for 4 hours in a hydrogen reductionatmosphere (preferably at a hydrogen partial pressure of 0.1 to 1atmospheres) at 550° C. or in an inert atmosphere (at atmosphericpressure) at 550° C. Here, in the region 300 on which the nickelsilicide film is selectively formed, crystallization of the crystallinesilicon film 304 occurs vertically with respect to the substrate 301.Also, in the regions other than the region 300, as shown by the arrow305, crystal growth occurs in a lateral direction from the region 300(parallel to the substrate).

As a result of the above mentioned steps, it is possible to obtain acrystalline silicon film by crystallization of an amorphous silicon film304. Next, a silicon oxide film 306 is formed by the sputtering methodto a thickness of 1000 Å as a gate insulation film. In the sputtering,silicon oxide is used as the target and the temperature of the substrateduring the sputtering is 200 to 400° C., for example 350° C., theatmosphere for sputtering consists of oxygen and argon, and theargon/oxygen ratio is 0 to 0.5, for example 0.1 or less. The elementsare then separated to ensure an active layer region for the TFT. Here,it is important that no front of crystal growth such as indicated by 305be present in the section which is to become the channel-forming region.In this manner it is possible to prevent influence of the elementalnickel on the carrier migrating between the source and drain in thechannel forming region.

Next, a film of aluminum (containing 0.1 to 2% silicon) is formed by thesputtering method, to a thickness of 6000 to 8000 Å, for example 6000 Å.

Also, the aluminum film is patterned to form gate electrodes 307, 309.Then, the surfaces of these aluminum electrodes are subjected toanodization to form oxide layers 308, 310. This anodization wasperformed in an ethylene glycol solution which contained 1 to 5%tartaric acid. The thickness of each of the resulting oxide layers 308,310 was 2000 Å. In the following ion doping step, these oxide layers 308and 310 are thick enough to form offset gate regions, and thus thelengths of the offset gate regions may be determined in the aboveanodization step.

Next, an impurity is added by the ion doping method (ion implantationmethod) to impart one conductive type to the active layer regions(making up the source/drain and the channel). In this doping step,impurities (phosphorus and boron) are implanted using the gate electrode307 and its surrounding oxide layer 308 and the gate electrode 309 andits surrounding oxide layer 310 as masks. The doping gas used isphosphine (PH₃) or diborane (B₂H₆), and in the former case theacceleration voltage is 60 to 90 kV, for example 80 kV, and in thelatter case it is 40 to 80 kV, for example 65 kV. The dose is 1×10¹⁵ to8×10¹⁵ cm⁻², for example 2×10¹⁵ cm⁻² of phosphorus and 5×10¹⁵ cm⁻² ofboron. During the doping, each of the elements is selectively doped bycovering the other region with a photoresist. As a result, N-typeimpurity regions 314 and 316 are formed, and P-type impurity regions 311and 313 are formed, and thus it is possible to form P-channel type TFT(PTFT) regions and N-channel type TFT (NTFT) regions.

Then, annealing is performed by irradiation with laser light. The laserlight used was from a KrF excimer laser (wavelength: 248 nm, pulsewidth: 20 nsec), but other lasers may be used. The conditions of thelaser light irradiation are an energy density of 200 to 400 mJ/cm², forexample 250 mJ/cm², and irradiation with 2 to 10 shots, for example 2shots per location. It is useful to heat the substrate to about 200 to450° C. at the time of laser irradiation. Since in this laser annealingprocess nickel is dispersed in the precrystallized regions, the laserlight irradiation readily promotes recrystallization, and the impurityregions 311 and 313 doped with P-type-imparting impurities and theimpurity regions 314 and 316 doped with N-type-imparting impurities maybe easily activated.

This step may be a method of lamp annealing using infrared rays (forexample, 1.2 μm). Infrared rays are readily absorbed by silicon and thuseffective annealing equal to thermal annealing at 1000° C. or higher maybe performed. On the other hand, since they are poorly absorbed by theglass substrate there is no high-temperature heating of the glasssubstrate and treatment may be finished within a short period of time,for which reasons this may be said to be the ideal method for processesin which there is shrinkage of the glass substrate.

Next, a silicon oxide film 318 of thickness 6000 Å is formed by theplasma CVD method as an interlayer insulator, contact holes are formedtherein, and TFT electrodes/wiring 317, 320, 319 are formed by amulti-layered film of a metallic material, for example, titanium nitrideand aluminum. Finally, annealing is performed at 350° C. for 30 minutesin a hydrogen atmosphere at 1 atmospheric pressure, to complete thesemiconductor circuit constructed with complementary TFTs (FIG. 10(D)).

The circuit described above has a CMOS structure with the PTFT and NTFTprovided in a complementary manner, but in the above process, two TFTsmay be simultaneously constructed and bisected to simultaneously preparetwo separate TFTs.

FIG. 11 shows an outline as seen from the top of FIG. 10(D). The symbolsin FIG. 11 correspond to those in FIG. 10. As shown in FIG. 11, thedirection of crystallization is in the direction shown by the arrow, andcrystal growth occurs in the direction of the source/drain regions(direction of a line between the source region and the drain region).During operation of a TFT with this construction, the carrier migratesalong the crystals which have grown in a needle-like or columnar mannerbetween the source and the drain. That is, the carrier migrates alongthe crystalline grain boundary of the needle-like or columnar crystals.Consequently, it is possible to lower the resistance undergone when thecarrier migrates, and obtain a TFT with a high degree of mobility.

In this example, the method employed to introduce the nickel was one inwhich nickel was used to selectively form a nickel film on theunderlying film 302 under the amorphous silicon film 304 (since the filmis very thin, it is not easily discernible as a film), and crystalgrowth was induced from that section, but the method may also be one inwhich a nickel silicide film is selectively formed after formation ofthe amorphous silicon film 304. That is, the crystal growth may beinduced from either the top surface or the bottom of the amorphoussilicon film. Furthermore, the method employed may also be one in whichan amorphous silicon film is formed in advance, and ion doping is usedto selectively implant nickel ion in the amorphous silicon film 304.This method is characterized in that the concentration of the elementalnickel may be controlled. Alternatively, the method may be plasmatreatment or CVD.

Example 10

This example is an case of an active-type liquid crystal displayprovided with N channel-type TFTs as switching elements for each of thepicture elements. The following is an explanation regarding a singlepicture element, but a plurality (usually several hundreds of thousands)of picture elements are formed with the same construction. Also, it neednot be mentioned that a P channel-type rather than an N channel-type maybe used. Furthermore, it may be used in the peripheral circuit sectioninstead of the picture element section of the liquid crystal display. Itmay also be employed in an image sensor or any other type of device. Inother words, there is no particular restriction on its use so long as itis used as a thin-film transistor.

An outline of the preparation steps for this example is shown in FIG.12. In this example a Corning 7059 glass plate (thickness 1.1 mm,300×400 mm) was used as the glass substrate 401. First, an underlyingfilm 402 (silicon oxide) is formed to a thickness of 2000 Å by thesputtering method. Then, for selective introduction of nickel, a mask403 is formed by a metal mask, a silicon oxide film, photoresist, or thelike. A nickel silicide film is also formed by the sputtering method.This nickel silicide film is formed by the sputtering method to athickness of 5 to 200 Å, for example 20 Å. The nickel silicide film hasa chemical formula of NiSix, where 0.4≦×≦2.5, for example ×=2.0. Thusthere is selectively formed a nickel silicide film over the region of404.

An amorphous silicon film 405 is then formed to a thickness of 1000 Å bythe LPCVD method or the plasma CVD method, and dehydrogenated at 400° C.for one hour, after which it is crystallized by thermal annealing. Theannealing process was conducted at 550° C. for 4 hours in a hydrogenreduction atmosphere (preferably with a hydrogen partial pressure of 0.1to 1 atmosphere). The thermal annealing process may also be carried outin an inert atmosphere of nitrogen or the like.

In this annealing process, since a nickel silicide film has been formedon the part of the region under the amorphous silicon film 405,crystallization starts at this section. During the crystallization, asshown by the arrow in FIG. 12(B), crystal growth of the siliconprogresses in a direction vertical to the substrate 401 at the section404 where the nickel silicide film has been formed. In addition, as alsoshown by another arrow, in the regions on which the nickel silicide filmhas not been formed (the regions other than region 405), crystal growthoccurs in a parallel manner with respect to the substrate.

In this manner it is possible to obtain a semiconductor film 405comprising crystalline silicon. Next, the above mentioned semiconductorfilm 405 is patterned to form an island semiconductor region (activelayer of the TFT). Here, it is important that no front of crystal growthsuch as indicated by the arrow be present in the active layer,particularly the channel-forming region. Specifically, if the frontsection indicated by the arrow in FIG. 12(B) is the end (front) of thecrystal growth, then it is useful to remove the crystalline silicon film405 at the section of nickel introduction 404 and the section at the endof the arrow (left edge of the drawing) by etching, and to use theintermediate sections of crystal growth of the crystalline silicon film405 in a direction parallel to the substrate as the active layer. Thisis based on the fact that the nickel is concentrated at the frontsections of the crystal growth, and is to prevent the adverse effects ofthe nickel concentrated at the front section on the characteristics ofthe TFT.

Also, a silicon oxide gate insulation film (thickness: 70 to 120 nmtypically 100 nm) 406 is formed by the plasma CVD method in an oxygenatmosphere, using tetraethoxysilane (TEOS) as the starting material. Thetemperature of the substrate is set to 400° C. or lower, and preferably200 to 350° C., to prevent shrinkage and warpage of the glass.

Next, a publicly known film consisting mainly of silicon is formed bythe CVD method, and it is patterned to form a gate electrode 407. Then,phosphorus is doped by ion implantation as an N-type impurity, and asource region 408, channel-forming region 409 and drain region 410 areformed in a self-aligning manner. Then it is irradiated with KrF laserlight to improve the crystallinity of the silicon film whosecrystallinity had been impaired by the ion implantation. Here the energydensity of the laser light is set to 250 to 300 mJ/cm². As a result ofthis laser irradiation, the sheet resistance of the source/drain of thisTFT is 300 to 800 Ω/cm ². The annealing step may also be effectivelycarried out by infrared lamp annealing.

Next, an interlayer insulator 411 is formed with silicon oxide, and apicture element electrode 412 is formed with an ITO. In addition,contact holes are formed therein, electrodes 413, 414 are formed in thesource/drain regions of the TFT using a chrome/aluminum multi-layerfilm, and one of the electrodes 413 is also connected to the ITO 412.Finally, annealing is performed in hydrogen at 200 to 300° C. for 2hours to complete the hydrogenation of the silicon. Thus, the TFT iscompleted. This process is carried out simultaneously for the otherpicture element regions.

The TFT prepared in this example uses a crystalline silicon film inwhich crystal growth has occurred in the direction of flow of thecarrier, as the active layer making up the source region,channel-forming region and drain region, and thus since the carriermigrates along the crystal grain boundary of the needle-like or columnarcrystals, without intersecting the crystal grain boundary, the resultingTFT has a carrier with a high degree of mobility. The TFT prepared inthis example was an N channel-type, and its degree of mobility was 90 to130 (cm²/Vs). Considering that the mobility of N channel-type TFTs usingcrystalline silicon obtained by crystallization with conventionalthermal annealing at 600° C. for 48 hours has been 80 to 100 (cm²/Vs),the improvement in the properties is notable.

Also, the degree of mobility of a P channel-type TFT prepared by amethod similar to the above process was measured and found to be 50 to80 (cm²/Vs). This is also a notable improvement in the properties,considering that the mobility of P channel-type TFTs using crystallinesilicon films obtained by crystallization with conventional thermalannealing at 600° C. for 48 hours has been 30 to 60 (cm²/Vs).

Example 11

This is a modification of the TFT in Example 10 which is provided with asource/drain in a perpendicular direction with respect to the directionof crystal growth. That is, it is a case with a construction in whichthe direction of migration of the carrier is perpendicular to thedirection of crystal growth, and thus the migration of the carrierintersects the crystal grain boundary of the needle-like or columnarcrystals. With this type of construction, the resistance between thesource and the drain may be increased. This is because the carrier mustmigrate so that it intersects the crystal grain boundary of theneedle-like or columnar crystals. To achieve the construction of thisexample, it is only necessary to determine the orientation of the TFT inthe construction in Example 10.

Example 12

The main aspect of this example lies in the fact that the orientation ofthe TFT in the construction in Example 10 (Here, the orientation isdefined by the connecting line between the source/drain regions. Thatis, the direction of the TFT is determined by the orientation of thecarrier flow.) is set at a desired angle with the direction of crystalgrowth of the crystalline silicon film with respect to the surface ofthe substrate, for selection of the properties of the TFT.

As described above, if the carrier is allowed to migrate in thedirection of the crystal growth, then it will migrate along the crystalgrain boundary, and consequently the degree of mobility thereof will beimproved. On the other hand, if the carrier is allowed to migrateperpendicularly with respect to the direction of the crystal growth,then the carrier must intersect multiple grain boundaries, and thus thedegree of mobility of the carrier will be reduced.

Here, through appropriate selection between these two conditions, thatis, by setting the angle between the direction of crystal growth and thedirection of migration of the carrier within a range of 0 to 90°, it ispossible to control the mobility of the carrier. Viewed differently, bysetting the above angle between the direction of crystal growth and thedirection of migration of the carrier, it becomes possible to controlthe resistance between the source and drain regions. Naturally, thisconstruction may also be used for the construction in Example 1. In thatcase, the slit-shaped region 400 of trace addition of nickel shown inFIG. 11 is rotated within a range of 0 to 90° for selection within 0 to90° of the angle between the direction of crystal growth shown by thearrow 405 and the line connecting the source and drain regions. Also,this angle may be set to near 0° to increase the mobility for aconstruction with a low degree of electrical resistance between thesource and drain regions. Furthermore, the angle may be set to near 90°to lower the mobility for a construction with a high degree ofresistance between the source and drain regions.

As mentioned above, for a TFT which employs a non-single crystal siliconsemiconductor film formed on a substrate and having crystallinityresulting from crystal growth parallel to the surface of the substrate,the direction of the flow of the carrier migrating in the TFT may bematched to the direction of crystal growth for a construction in whichthe migration of the carrier is along (parallel to) the crystal grainboundary of the needle-like or columnar crystals, to obtain a TFT with ahigh degree of mobility.

Furthermore, since the metal catalyst for promotion of crystallizationis concentrated at the front sections of the crystal growth parallel tothe substrate, a TFT may be formed without using these regions, toincrease the operational stability and reliability of the TFT. Inaddition, by making a semiconductor device using a crystalline siliconfilm prepared by introduction of a catalytic element for short timecrystallization at a low temperature followed by irradiation with laserlight or other intense light, it is possible to obtain a device with ahigh degree of productivity and favorable characteristics.

What is claimed is:
 1. A semiconductor device comprising: a crystallinesemiconductor film comprising silicon provided over a substrate andcontaining a crystallization-promoting element, wherein the crystalgrowth face is the face (111).
 2. The device of claim 1 wherein thecrystallization-promoting element comprises one or more elementsselected from the group consisting of Ni, Pd, Pt, Cu, Ag, Au, In, Sn,Pb, As and Sb.
 3. The device of claim 1 wherein thecrystallization-promoting element comprises one or more elementsselected from the group consisting of Group VIII, Group IIIb, Group IVband Group Vb elements.
 4. The device of claim 1 wherein concentration ofthe crystallization-promoting element is 1×10¹⁶ atoms/cm³ to 1×10¹⁹atoms/cm³.
 5. A semiconductor device comprising: a crystallinesemiconductor film comprising silicon provided over a substrate andcontaining a crystallization-promoting element, wherein (111)orientation ratio of the semiconductor films is 0.33 to
 1. 6. The deviceof claim 5 wherein the crystallization-promoting element comprises oneor more elements selected from the group consisting of Ni, Pd, Pt, Cu,Ag, Au, In, Sn, Pb, As and Sb.
 7. The device of claim 5 wherein thecrystallization-promoting element comprises one or more elementsselected from the group consisting of Group VIII, Group IIIb, Group IVband Group Vb elements.
 8. The device of claim 5 wherein concentration ofthe crystallization-promoting element is 1×10¹⁶ atoms/cm³ to 1×10¹⁹atoms/cm³.
 9. A semiconductor device comprising: a crystallinesemiconductor film comprising silicon provided over a substrate andcontaining a crystallization-promoting element, wherein (111)orientation ratio of the semiconductor film is 0.67 to
 1. 10. The deviceof claim 9 wherein the crystallization-promoting element comprises oneor more elements selected from the group consisting of Ni, Pd, Pt, Cu,Ag, Au, In, Sn, Pb, As and Sb.
 11. The device of claim 9 wherein thecrystallization-promoting element comprises one or more elementsselected from the group consisting of Group VIII, Group IIIb, Group IVband Group Vb elements.
 12. The device of claim 9 wherein concentrationof the crystallization-promoting element is 1×10¹⁶ atoms/cm³ to 1×10¹⁹atoms/cm³.